1. Field of the Invention
The present invention relates to solid-state image detectors and particularly radiology image detectors receiving X radiation. The invention is also of interest in the field of non-destructive inspection in which the radiation used does not always consist of X-rays but of gamma rays, for example. From now on, the term ionising radiation is used.
2. Discussion of the Background
During the acquisition of an image of a body, whether this is an object or a patient, subjected to incident ionising radiation, X radiation for example, via an image detector, it is normal to measure the radiation dose transmitted by the body to the detector during the exposure, so as to ensure that the detector is actually working in an optimal operating range. This measurement makes it possible, if appropriate, to monitor the duration of the exposure. The measurement should obviously not disturb the image of the body picked up by the detector.
In the conventional case of medical radiological imaging, the image detector is formed by a radiological film. An X-ray generator bombards a part of the patient and the X-rays, emerging from this part to be examined, register on the radiological film to which a fluorescent screen can be applied, allowing the image to be viewed.
It is conventional to place ionising chambers on the path of the X-rays between the patient and the film in order to analyse the radiation received by the film in several regions judged typical of the image.
The intensity of the X radiation received by the ionising chambers is integrated over a few square centimetres in order to give an average measurement of exposure in these regions. This measurement makes it possible to make best use of the dynamic range of the image detector and to cut off the voltage to the X-ray generator when the optimal dose is reached.
These ionisation chambers absorb very little of the X-rays and their shadow is not visible on conventional image detectors.
Image detectors with no radiological film, using radiological image intensifier (RII) tubes, have also been developed. The incident ionising radiation which comes out of the body to be examined penetrates into the RII tube where it is converted by a scintillator into lower-energy radiation, generally light. This light is then converted into electrons by a photocathode. The electrons come to bombard a luminescent screen on which can be viewed a visible image which is the converted image of the ionising radiation which has not been absorbed by the body. In this type of device, monitoring of the exposure takes place directly by measuring the photocathode current.
The new image detectors which are in the process of being developed are solid-state detectors formed by a plurality of sensitive elements generally arranged in a matrix.
These solid-state detectors are produced by depositing a thin film of a semiconductor material, for example amorphous silicon, on an insulating support.
If the sensitive elements are photosensitive, that is to say sensitive to visible or near-visible radiation, a scintillator is interposed between the body and the detector in order to convert the incident ionising radiation which has passed through the body into visible or near-visible radiation. The photosensitive elements react to this visible or near-visible radiation.
The scintillator may be made of caesium iodide, for example. The photosensitive elements then carry out photoelectric conversion of the radiation which they receive into electrical signals which can be made use of by appropriate electronic circuits.
If the sensitive elements are sensitive to charges, a radioconductive layer is interposed between the body and the detector. It converts the incident ionising radiation into charges and the sensitive elements of the detector, receiving these charges, convert them into electronic signals which can be made use of by appropriate electronic circuits.
These solid-state image detectors are very promising, since they supply a digital image. The image can be viewed in real time, easily stored, recovered, processed, transmitted to another site, etc.
These solid-state image detectors open up the possibility of viewing very faintly contrasting details which were not visible on conventional radiological films. The images obtained are much sharper and more detailed.
With these solid-state image detectors, there is no longer any question of using ionising chambers since their shadow is superimposed on the image and is now visible and intrusive.
The present invention therefore proposes an exposure-measuring device which is capable of being used with a solid-state image detector, formed from sensitive elements exposed to ionising radiation originating from a body to be observed. This measuring device does not disturb the image.
More precisely, the invention is a device for measuring the exposure of a solid-state image detector having a first face exposed to ionising radiation representative of the image and letting unabsorbed ionising radiation exit through another face opposite the first one. The measuring device is intended to be placed close to the other face and to be exposed to this unabsorbed ionising radiation. It includes at least one optical fibre emitting visible or near-visible radiation, obtained by conversion in the optical fibre, towards a detection device, this visible or near-visible radiation being representative of the unabsorbed ionising radiation, the detection device producing a signal representative of the exposure of the image detector.
According to a first configuration of the invention, the optical fibre is scintillating and has its lateral surface exposed to the unabsorbed ionising radiation, it converts the unabsorbed ionising radiation into the visible or near-visible radiation emitted towards the detection device.
According to another configuration of the invention, the measuring device includes a converter of the unabsorbed ionising radiation into a second visible or near-visible radiation. The optical fibre the lateral surface of which is exposed to the second visible or near-visible radiation is fluorescent and converts the second visible or near-visible radiation into the visible or near-visible radiation emitted towards the detection device.
In a simple first embodiment, the converter includes at least one scintillator screen. The scintillator screen may be placed between the image detector and the optical fibre or opposite the image detector with respect to the optical fibre.
In another embodiment giving good conversion efficiency because of better optical coupling, the converter may include agglomerated powder of a scintillating material which sheaths the optical fibre.
In another embodiment having substantially equivalent performance, the converter may include a varnish loaded with scintillating material coating the optical fibre.
It may be that only one of the ends of the optical fibre is coupled to the detection device and, in this case, it is preferable for the other end of the optical fibre to be reflecting.
It is possible for the two ends of the optical fibre to be coupled to the detection device. In this case, the detection device may include a single detector supplying the signal representative of the exposure, this detector being illuminated by the two ends of the optical fibre. It may also include two detectors, each being illuminated by one end of the optical fibre and supplying a signal, the two signals being combined to give the signal representative of the exposure.
When the solid-state image detector is photo-sensitive and includes a zero-reset illumination system, on the same side as its other face, the illumination system may be situated opposite the image detector with respect to the scintillating optical fibre or else between the scintillating optical fibre and the image detector.
When the solid-state image detector is photo-sensitive and includes a zero-reset illumination system, on the same side as its other face, the illumination system is placed between the fluorescent optical fibre and the image detector as the optical fibre is sensitive to the illumination.
It is preferable for the optical fibre to follow an appropriate pattern so as to occupy a significant surface area facing the image detector. It may be wound into a spiral or series of bends.
The present invention also relates to a solid-state image detector including a plurality of sensitive elements, which is intended to detect the image of a body, exposed to incident ionising radiation originating from the body which is equipped with at least one exposure-measuring device as characterized above.